The bushings of the present invention are used to connect high voltage lines to padmounted and subsurface electrical switchgear. Such switchgear are typically mounted within sealed housings and insulated with oil. The bushings used in these applications are typically bolted to the external surface of the wall of the switchgear housing and can easily be removed for repair or replacement. Recently, the electrical distribution industry has begun to use padmounted and subsurface switchgear in which sulfur hexafiouride (SF.sub.6) replaces oil as the insulating medium. SF.sub.6 gas is preferable to oil in many instances where safety is an issue, in part because it decreases the risk of explosion. Because SF.sub.6 is less insulating than oil along surfaces of solid insulators, bushings used in SF.sub.6 -insulated applications must have greater resistance to arcing than those used in oil-insulated applications. Arcing can occur either through the medium surrounding the components, or can occur across the surface of the components. The latter phenomenon is called are tracking.
When SF.sub.6 is used as the insulating medium, however, greater precautions must be taken to prevent the gas from escaping, because its presence is necessary primarily to preserve the insulation integrity of the switchgear. Bushings designed for use in SF.sub.6 applications are typically designed to be welded to the bushing housing in order to form an unbroken joint and thereby eliminate the need for elastomeric seals, which can be affected by the insulating gas. Presently, bushings designed to be welded to the housing wall have several disadvantages. First, because it is not practical to un-weld and re-weld a bushing in the field, the entire switchgear is typically removed to an operating facility when a connection needs to be replaced or repaired. This is costly and inefficient. Second, weld-mounted bushings typically include an integral metal flange molded into the bushing body, for welded attachment to the housing wall. The inclusion of this flange in the bushing body adds complexity to the manufacture of molded bushings, with the result that welded bushings are not available in as many configurations as are bolt-mounted bushings.
In contrast, bolted-mounted bushings designed for use with oil-insulated switchgear are in abundant supply and are relatively easy to manufacture and install. However, when a bolt-mounted bushing is used in a gas-insulated environment, the dielectric properties of the bushing are generally not adequate to withstand the voltage stress and dielectric breakdown results. Specifically, the shank of the bushing is shorter and includes a shielding layer of semiconductive material around its middle. The inner edge of the shielding layer is the site of significant voltage stress. Hence, it is desired to provide a bolt-mounted bushing that is simple and inexpensive to manufacture and install, yet suitable for use in gas-insulated switchgear.